By far the best reason to go to Newark, New Jersey is if you are flying into New York City's third largest, and arguably nicest, international airport. The city of Newark itself is the sort of place even those who live there would often like to avoid, so they don't get many visitors who intend to stick around to see the sights. Years ago New Jersey itself had an unfair reputation of being a terrible place, and that was certainly perpetuated by the industrial wasteland that lies between Newark's airport and New York City itself. The odd thing is that nearly the entire rest of the state is gorgeous and forested, so the fact that most visitors see only by far its worst section is a shame.
What to do
The first thing most visitors to Newark do is go straight to New York City and you couldn't be blamed for following the herd. As mentioned above, New Jersey has many lovely towns and areas, some close by, but this isn't one of them. There are a few interesting things to do if you are really interested, but beware that parts of the city center of Newark aren't really safe, especially after dark.
If you are still with us then you might be interested in the Newark Museum, which incorporates art, science, a mini zoo, a planetarium, and several other things. It's probably more of a field trip destination for school children, but it's housed in a restored 1885 mansion that is a National Historic Landmark.
You can take a train if you are coming from Washington DC or New York City, but nearly everyone will want to book a flight to Newark through the large Newark Liberty International Airport (code: EWR). It's usually fairly easy to get a cheap flight there because competition is heavy, even from many international airlines.
Where to stay
If you are coming to visit New York City you'll certainly be more interested in a New York Hotel. If you want or need to stay near the airport itself there is no shortage of Newark hotels, and some of them are much less expensive than hotels in Manhattan or near the other NYC airports.